Mechanically, an oscillatory movement is damped by the friction forces on the bearings, the guiding surfaces and/or the medium in which moves the oscillating member. Said movement therefore has to be sustained by supplying it with energy compensating for that absorbed by the friction forces. Usually, said energy is provided in the well known clockworks by a so-called escapement or by an electromagnetic coil fed at the oscillation frequency. The characteristic of such known mechanisms for sustaining a pendular motion is that the transmission of the work providing the compensation for the friction occurs substantially in the portion of the trajectory which is in the vicinity of the point of equilibrium of the device and is cancelled in the vicinity of the dead points so as to let the device oscillate at its own frequency. It will be recalled that in an oscillating device such as hereabove, the return forces towards the point of equilibrium are maximum at the dead points corresponding to a maximum deviation, and that the speeds are highest when the oscillating device passes by the point of equilibrium while the return force is cancelled at this point. In order to transmit energy sustaining the oscillatory movement of an oscillating device in the area of the point of equilibrium, the application speed of the so-called sustaining force must therefore be high and at least equal to the instantaneous speed of the receiving member which is rigidly connected to the oscillating mass. In an escapement mechanism, the peripheral speed of the wheel at the point of contact with the pendulum finger of the escapement tends to be higher than the instantaneous oscillation speed of said finger.
In the devices of the free stroke type, there is exerted on the mass, while it moves at a low speed over the first part of its trajectory from a dead point, a force which is added to that exerted by the storing and restitution device of the kinetic energy.
Moreover, one should remark that if a work other than that resulting from the friction forces is required from the oscillating device, the oscillatory movement goes on only if it is sustained by introducing in the device energy equivalent to that which has been performed.
On the other hand, it is known how to use the kinetic energy of a pendulum mass for performing work, for example in pendular rams, but this mode of utilization does not correspond to a sustained oscillatory movement.